x1 is x2 (li; no default value assumed, but - unless explicitly overridden - assumed to be expressed with significant figures) years old according to standard of measurement x3; colloquial form: x1 is/has x2 (years (of age)).
At the referenced time (specified by ca or the like), x1's life/existence is an extended event which is thus far x2 years long in duration, as measured according to standards x3 (which might include starting and ending points, choice of frame of reference, ignoring of suspended animation, the definition of the unit 'year' and the use of calendars or continual time, etc.). As a measured quantity, x2 is assumed to be expressed in significant figure form; this feature may be explicitly overridden (possibly with zmico). There are other ways to express this concept, of course, but this word is meant for convenience and brevity, while maintaining clarity (even if not, entirely, formality); it is, essentially, elapsed time of the existence of a being/object in its current state of functionality as measured in years (so, this word is, basically, a more specific form of "tu'a x1 nanca"). See also: detmive.